Interstellar Silicon Depletion and the Ultraviolet Extinction

Abstract

Abstract Spinning small silicate grains were recently invoked to account for the Galactic foreground anomalous microwave emission. These grains, if present, will absorb starlight in the far-ultraviolet (UV). There is also renewed interest in attributing the enigmatic 2175 Å interstellar extinction bump to small silicates. To probe the role of silicon in the UV extinction, we explore the relations between the amount of silicon required to be locked up in silicates [ Si / H ] dust and the 2175 Å bump or the far-UV extinction rise, based on an analysis of the extinction curves along 46 Galactic sightlines for which the gas-phase silicon abundance [ Si / H ] gas is known. We derive [ Si / H ] dust either from [ Si / H ] ISM − [ Si / H ] gas or from the Kramers–Kronig relation, which relates the wavelength-integrated extinction to the total dust volume, where [ Si / H ] ISM is the interstellar silicon reference abundance and taken to be that of proto-Sun or B stars. We also derive [ Si / H ] dust from fitting the observed extinction curves with a mixture of amorphous silicates and graphitic grains. We find that in all three cases [ Si / H ] dust shows no correlation with the 2175 Å bump, while the carbon depletion [ C / H ] dust tends to correlate with the 2175 Å bump. This supports carbon grains instead of silicates as the possible carriers of the 2175 Å bump. We also find that neither [ Si / H ] dust nor [ C / H ] dust alone correlates with the far-UV extinction, suggesting that the far-UV extinction is a combined effect of small carbon grains and silicates.